Production of acetic acid with high conversion rate

ABSTRACT

The present invention is to a process for producing acetic acid comprising the step of reacting a carbon monoxide feed and methanol and/or a methanol derivative in a first reactor to produce a crude acetic acid product. The carbon monoxide may comprise less than 99.5 mol % carbon monoxide. The process further comprises the step of purging a carbon monoxide purge stream comprising a first amount of residual carbon monoxide and one or more impurities. Preferably, the first amount is greater than 20 mol %. The process further comprises the step of separating the crude acetic acid product into at least one derivative stream comprising a second amount of residual carbon monoxide. The process further comprises the step of reacting at least a portion of the second amount of residual carbon monoxide and methanol and/or a methanol derivative in a second reactor to produce additional acetic acid.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part if and claims priority toU.S. application Ser. No. 12/892,348, filed on Sep. 28, 2010, thedisclosure of which is hereby incorporated by reference.

FIELD OF THE INVENTION

This invention relates to processes for producing acetic acid fromcarbon monoxide and in particular to improved processes that reactresidual carbon monoxide to form additional acetic acid, thus improvingoverall carbon monoxide efficiency.

BACKGROUND OF THE INVENTION

A widely used and successful commercial process for synthesizing aceticacid involves the catalyzed carbonylation of methanol with carbonmonoxide. Catalysts used in this reaction typically contain rhodiumand/or iridium as well as a halogen promoter, for example methyl iodide.The carbonylation reaction may be conducted by continuously bubblingcarbon monoxide through a liquid reaction medium in which the catalystis dissolved. In addition to the catalyst, the reaction medium also maycomprise methyl acetate, water, methyl iodide and the catalyst.Conventional commercial processes for carbonylation of methanol includethose described in U.S. Pat. Nos. 3,769,329, 5,001,259, 5,026,908, and5,144,068, the disclosures of which are hereby incorporated byreference. Also, U.S. Pat. No. 6,617,471, the disclosure of which ishereby incorporated by reference, discloses a vapor-phase carbonylationmethod for producing esters and carboxylic acids from reactantscomprising lower alkyl alcohols, lower alkyl alcohol generatingcompounds, and mixtures thereof. Another conventional methanolcarbonylation process includes the Cativa™ process, which is discussedin Jones, J. H. (2002), “The Cativa™ Process for the Manufacture ofAcetic Acid,” Platinum Metals Review, 44 (3): 94-105, the disclosure ofwhich is hereby incorporated by reference.

In the methanol carbonylation reaction, carbon monoxide, by-productgases, and feed impurities, e.g., hydrogen, nitrogen, argon, methane,and carbon dioxide, that are initially present in the carbon monoxidefeed may build up in the reactor as the crude acetic acid product isformed. To improve overall reaction and catalyst efficiency and toprevent the build-up of these gases, a purge stream, e.g., an off-gasstream, is typically vented from the reactor. The vented purge streammay comprise carbon monoxide, inerts, volatile halogen promoters, aceticacid, water, unreacted methanol, methyl acetate and/or feed impurities.Typically, the purge stream as a whole is withdrawn directly from thereactor, e.g., the purge stream components are not withdrawnindividually. Because the carbon monoxide in the purge stream is ventedfrom the process, the same is not converted to acetic acid. As such,this loss of carbon monoxide contributes to a reduction in a carbonmonoxide efficiency of the system. In addition to the carbon monoxidelost via the purge stream, residual carbon monoxide that remains in thecrude acetic acid product after the primary reaction often is separatedand purged, which further contributes to reductions in carbon monoxideefficiency. Accordingly, to improve overall carbon monoxide efficiency,conventional methanol carbonylation systems have attempted to reduce thesize of the vented purge stream and the amount of carbon monoxidetherein.

Even though the sizes of conventional purge streams are often minimized,the purge stream still may be processed to recover and/or utilize thecomponents thereof. For example, the purge stream, once withdrawn, maybe processed to recover volatile halogen promoters, acetic acid, water,unreacted methanol, and/or methyl acetate, which may in turn be recycledto the reactor. As another example, the small amount of carbon monoxidein the vent stream may be used to enhance catalyst stability, e.g., toreduce catalyst precipitation, in the units of the system.

As another example, U.S. Pat. No. 5,917,089 discloses that a purgestream, e.g., “off-gas” from the reactor may be fed directly to a secondreactor, along with fresh methanol, to produce additional carbonylationproduct, i.e. acetic acid. The off-gas, as known in the art, is not aderivative stream. Although the carbon monoxide in the purge stream maybe converted and efficiency may be slightly improved in this case, theamount of carbon monoxide available for catalyst stabilization islessened.

Also, as noted above, conventional carbon monoxide feed impurities arealso vented via the off-gas. Because the off-gas stream is vented fromthe reactor as a whole, conventional processes keep the amount ofimpurities in the carbon monoxide feed stream as low as possible, e.g.,to minimize the loss of carbon monoxide that would accompany theunwanted impurities that are purged. Accordingly, conventionalcarbonylation processes often employ high purity carbon monoxide feedstreams, which contain very low amounts of impurities. As a result ofthe very low amount of impurities, only smaller amounts of the off gasneed to be vented from the reactor.

Generally speaking, the crude acetic acid product from the reactor isthen processed via a purification train to remove impurities and providea high quality acetic acid product. The purification train may includeseveral vents, which purge non-condensable gases. The gases that arepurged via the purification train vents may be processed in a recoveryunit to recover light boiling point components, such as the halogenpromoter, as described in US Publication No. 2008/0293966, the entirecontent and disclosure of which is hereby incorporated by reference. Inmost cases, at least a portion of the non-condensable gases, e.g.,carbon monoxide, whether or not they pass through the recovery unit, aretypically purged or flared. As noted above, this loss of residual carbonmonoxide contributes to additional reductions in carbon monoxideefficiency.

Accordingly, in view of these references, the need exists for a methanolcarbonylation process that provides 1) an increased amount carbonmonoxide available for use as a catalyst stabilizer; and 2) an abilityto use a lower purity carbon monoxide feed, while maintaining a highoverall carbon monoxide efficiency.

SUMMARY OF THE INVENTION

The present invention is to process for producing acetic acid. In afirst embodiment, a process comprises the step of reacting (i) a carbonmonoxide feed; and (ii) at least one of methanol and a methanolderivative in a reactor, under conditions effective to produce a crudeacetic acid product. In preferred embodiments, the carbon monoxide feedis a lower purity carbon monoxide feed, e.g., a carbon monoxide feedcomprising less than 99.5 mol % carbon monoxide. For example, the carbonmonoxide feed may comprise from 20 mol % to 99.5 mol % carbon monoxide.The process, in some embodiments, further comprises the step of purgingfrom the first reactor a carbon monoxide purge stream comprising a firstamount of residual carbon monoxide and one or more impurities.Preferably, the first amount is greater than 20 mol %, based on thetotal moles of the purge stream. The process further comprises the stepof separating the crude acetic acid product into at least one derivativestream. At least one of the derivative stream(s) comprises residualcarbon monoxide, e.g., a second amount of residual carbon monoxide. Theprocess further comprises the step of reacting (i) at least a portion ofthe second amount of residual carbon monoxide; and (ii) at least one ofmethanol and a methanol derivative in a second reactor under conditionseffective to produce additional acetic acid. As a result of these steps,the inventive process achieves an overall carbon monoxide conversion ofat least 95%, based on the total amount of carbon monoxide in the carbonmonoxide feed.

In another embodiment, the process comprises the step of reacting (i)the first and second amounts of residual carbon monoxide; and (ii) atleast one of methanol and a methanol derivative, in a second reactorunder conditions effective to produce additional acetic acid. As aresult, in some embodiments, less than 10 mol % of the first and secondamounts of residual carbon monoxide, in total, are vented from theprocess.

BRIEF DESCRIPTION OF DRAWINGS

The invention is described in detail below with reference to theappended drawings, wherein like numerals designate similar parts.

FIG. 1 is a schematic diagram of an exemplary high pressure liquid phasecarbonylation secondary reactor for processing a purged derivativestream from an acetic acid purification section in accordance with anembodiment of the present invention.

FIG. 2 is a schematic diagram of an exemplary low pressure gas phasecarbonylation secondary reactor for processing a purged derivativestream from an acetic acid purification section in accordance with anembodiment of the present invention.

FIG. 3 is a detailed schematic diagram of an exemplary high pressureliquid phase carbonylation secondary reactor for processing a purgedderivative stream from an acetic acid purification section in accordancewith an embodiment of the present invention.

FIG. 4 is a detailed schematic diagram of an exemplary low pressure gasphase carbonylation secondary reaction for processing a purgedderivative stream from an acetic acid purification section in accordancewith an embodiment of the present invention.

FIG. 5 is a schematic diagram of an exemplary acetic acid reactionprocess, which includes reaction and separation, in accordance with anembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Introduction

The present invention generally relates to producing acetic acid fromresidual or unreacted carbon monoxide that is present either inderivative streams of a crude acetic acid product or in purge streams,e.g., off-gas streams, which are purged from a primary reactor. Thederivative stream(s) may be obtained during the purification of thecrude acetic acid product. Conventionally, the residual carbon monoxidein the derivative stream(s) is vented, purged and/or goes unutilized. Asa result of the present invention's ability to convert residual carbonmonoxide into additional acetic acid, higher overall carbon monoxideefficiencies may be obtained. In some embodiments, because the residualcarbon monoxide is converted and is not wasted, it is possible to purgelarger carbon monoxide-containing purge stream(s) from the firstreactor, e.g., the primary reactor. Preferably, these purge streamscomprise a higher volume percentages of carbon monoxide. As a result ofthe larger purge stream(s), alone or in combination with the derivativestream(s), the inventive processes provide availability of largeramounts of carbon monoxide for use throughout the carbonylation process,e.g., for catalyst stabilization. These larger amounts of carbonmonoxide are achieved without feeding additional carbon monoxide to thesystem and while still achieving overall carbon monoxide efficienciesequal to or greater than those of conventional carbonylation systems. Inaddition, because at least a portion of the residual carbon monoxide isconverted, the inventive process allows the use of a carbon monoxidefeed stream that comprises more impurities than a conventional feedstream. In these cases again, the overall carbon monoxide efficienciesremain equal to or greater than those of conventional carbonylationsystems. For example, the inventive carbon monoxide feed streams maycomprise less than 99.5 mol % carbon monoxide, e.g., less than 95 mol %or less than 90 mol %. These less pure feeds may allow significantflexibility in raw material procurement.

For purposes of the present application, a separation zone refers to theportion of the process that purifies the crude acetic acid productand/or separates impurities from the crude acetic acid product. In apreferred embodiment, the derivative streams are vented gas streams ofthe separation zone. The derivative streams exclude conventional“off-gas” streams that are vented from the reactor. In one embodiment,the present invention advantageously increases the overall carbonmonoxide efficiency by utilizing the residual carbon monoxide in thederivative streams to form additional acetic acid. Another embodiment ofthe present invention advantageously reduces the amount of carbonmonoxide that is purged or flared, e.g., from the purification andseparation section.

The present invention relates to a process for producing acetic acid. Inone embodiment, the process comprises the step of reacting (i) a carbonmonoxide feed; and (ii) at least one of methanol and a methanolderivative in a reactor, under conditions effective to produce a crudeacetic acid product. In one embodiment, the reactor is a first reactor,e.g., a primary reactor. The inventive system may further compriseadditional reactor(s), e.g., secondary reactor(s). In preferredembodiments, the carbon monoxide feed is a lower purity carbon monoxidefeed (as compared to a conventional carbon monoxide feed stream), e.g.,a carbon monoxide feed comprising less than 99.5 mol % carbon monoxide,e.g., less than 95 mol %, less than 90 mol % or less than 60 mol %. As alower limit, the carbon monoxide feed may comprise more than 10 mol %carbon monoxide, e.g., more than 20 mol %, more than 50 mol % or morethan 75 mol %. In terms of ranges, the carbon monoxide feed may comprisefrom 10 mol % to 99.5 mol % carbon monoxide, e.g., from 50 mol % to 95mol %, from 75 mol % to 90 mol %, or from 60 mol % to 70 mol %. Thelower purity carbon monoxide feeds of the present invention compriselower amount of carbon monoxide and/or higher amounts of impurities thanconventional carbon monoxide feeds. As such, the lower purity carbonmonoxide feed streams of the present invention may advantageouslypresent a wide array of previously unavailable procurement options,e.g., lower pricing and increased availability. For example, the carbonmonoxide feed streams of the present invention may come from a wastevent stream of hydroformylation, e.g., to form aldehydes from alkenes;from homologation, e.g., to form higher alcohols; from Fischer-Tropsche.g., to form liquid hydrocarbons processes; from a waste stream from asteel manufacturing process, from gasified biomass, and/or from andother waste stream, e.g., a low sulfur content waste stream. The presentinvention may employ such lower purity feed streams and still achieveacceptable carbon monoxide efficiencies.

In addition to carbon monoxide, the carbon monoxide feed of the presentinvention, in some embodiments, further comprises impurities. Exemplaryfeed impurities include hydrogen, nitrogen, argon, methane, and carbondioxide. This listing is not exclusive and other impurities may bepresent. In one embodiment, the carbon monoxide feed stream compriseslow amounts of sulfur, e.g., less than 1 mol %, less than 0.1 mol %, orless than 0.01 mol %. In one embodiment, the carbon monoxide feed streamis sulfur-free. Preferably, the carbon monoxide feed comprises at least0.5 mol % impurities, e.g., at least 5 mol % or at least 10 mol %. As anupper limit, the carbon monoxide feed may comprise less than 80 mol %impurities, e.g., less than 70 mol % or less than 50 mol %. In terms ofranges, the carbon monoxide feed may comprise from 0.5 mol % to 80 mol %impurities, e.g., from 5 mol % to 70 mol % or from 10 mol % to 50 mol %.In one embodiment, the carbon monoxide feed stream comprises from 30 mol% to 50 mol % impurities and from 50 mol % to 70 mol % carbon monoxide.One example of such a stream is a waste stream from a steelmanufacturing process.

The process, in some embodiments, further comprises the step of purgingfrom the first reactor a carbon monoxide purge stream comprising (i) afirst amount of residual carbon monoxide and (ii) one or moreimpurities. Preferably, the first amount of residual carbon monoxide isat least 20 mol % residual carbon monoxide, e.g., at least 30 mol % orat least 70 mol %. As an upper limit, the first amount of residualcarbon monoxide is less than 95 mol % carbon monoxide, e.g., less than50 mol % or less than 20 mol %. In terms of ranges, the first amount ofresidual carbon monoxide may range from 20 mol % to 95 mol % carbonmonoxide, e.g., from 30 mol % to 50 mol % Impurities are also present inthe purge stream. In one embodiment, the purge stream comprises from 0.5mol % to 80 mol % impurities, e.g., from 5 mol % to 70 mol % or from 10mol % to 50 mol %. In one embodiment, the purge stream comprises from0.5 mol % to 50 mol % hydrogen, e.g., from 1 mol % to 30 mol % or from 1mol % to 10 mol %. In one embodiment, the purge stream comprises from0.5 mol % to 20 mol % nitrogen, e.g., from 1 mol % to 10 mol %. In oneembodiment, the purge stream comprises from 0.5 mol % to 20 mol % argon,e.g., from 1 mol % to 10 mol %. In one embodiment, the purge streamcomprises from 0.5 mol % to 20 mol % methane, e.g., from 1 mol % to 10mol %. In one embodiment, the purge stream comprises from 0.5 mol % to30 mol % carbon dioxide, e.g., from 5 mol % to 25 mol %. In oneembodiment, the purge stream comprises low amounts of sulfur, e.g., lessthan 1 mol %, less than 0.1 mol %, or less than 0.01 mol %. In oneembodiment, the purge stream is sulfur-free. The purge stream maycomprise impurities in concentrations similar to those of the carbonmonoxide feed stream.

In a preferred embodiment, the purging maintains a carbon monoxidepartial pressure in the reactor at a predetermined level ranging from0.13MPa to 1.1 MPa, e.g., from 0.45 MPa to 0.85 MPa or from 0.65 MPa to0.80 MPa. In terms of lower limits, the purging may maintain a carbonmonoxide partial pressure in the reactor above 0.0.13 MPa, e.g., above0.45 MPa or above 0.65 MPa. In terms of upper limits, the purging maymaintain a carbon monoxide partial pressure in the reactor below 1.1MPa, e.g., below 0.85 MPa, or below 0.80 MPa.

The crude acetic acid product comprises acetic acid and residual carbonmonoxide. The residual carbon monoxide may be dissolved and/or entrainedin the crude acetic acid product. In one embodiment, the crude aceticacid product comprises residual carbon monoxide in an amount less than20 mol %, e.g., less than 10 mol %, less than 5 mol %, or less than 3mol %. In another embodiment, a flashed vapor phase crude acetic acidproduct has a carbon monoxide partial pressure that is less than 20% ofthe total pressure of the flashed crude acetic acid product, e.g., lessthan 10%, less than 5%, or less than 3%. In another embodiment, theflashed crude acetic acid product is at a total pressure of 0.3 MPa hasa carbon monoxide partial pressure of less than 0.06 MPa, e.g., lessthan 0.03 MPa; less than 0.015 MPa; or less than 0.009 MPa. Of course,for carbon monoxide to be reacted with methanol and/or a methanolderivative to form additional acetic acid, some amount of carbonmonoxide should be present. For example, the crude acetic acid productmay comprise residual carbon monoxide in an amount greater than 0.1 mol%, greater than 0.5 mol % or greater than 1 mol %. In terms of partialpressures, the flashed crude acetic acid product may have a (residual)carbon monoxide partial pressure of at least 0.1% of the total pressureof the flashed crude acetic acid product, e.g., at least 0.5% or atleast 1%. For example, where the flashed crude acetic acid product is at0.3 MPa total pressure, the carbon monoxide partial pressure may be ofat least 0.0003 MPa, e.g., at least 0.0015 MPa or at least 0.003 MPa. Inaddition, the crude acetic acid product may comprise at least 50 mol %acetic acid, e.g., at least 75 mol %, at least 90 mol %, at least 95 mol%, or at least 98 mol %. In other embodiments, the crude acetic acidproduct may have an acetic acid partial pressure of at least 50% of thetotal pressure of the crude acetic acid product, e.g., at least 75%, atleast 90%, at least 95%, or at least 98%. In terms of ranges, the crudeacetic acid product optionally comprises from 0.1 mol % to 20 mol %residual carbon monoxide, e.g., from 0.5 mol % to 10 mol %, or from 1mol % to 5 mol %; or from 50 mol % to 99.9 mol % acetic acid, e.g., from60 mol % to 99 mol %, or from 75 mol % to 95 mol %. In terms of partialpressures, the crude acetic acid product optionally has a carbonmonoxide partial pressure of from 0.1% to 20% of the total pressure ofthe crude acetic acid product, e.g., from 0.5% to 10% or from 1% to 5%;and an acetic acid partial pressure of from 50% to 99.9% of the totalpressure of the crude acetic acid product, e.g., from 60% to 99% or from75% to 95%. Optionally, the crude acetic acid product further comprisesmethyl iodide (liquid and/or vapor), methyl acetate, propionic acid,water, residual catalyst, and acetaldehyde. In one embodiment, the crudeacetic acid product may comprise acetic acid, residual catalyst,dissolved and/or entrained carbon monoxide, methyl iodide, methylacetate, water, permanganate reducing compounds (“PRCs”), and/or otherdissolved gases such as carbon dioxide, hydrogen, and methane.

In one embodiment, the process further comprises the step of separatingthe crude acetic acid product into at least one derivative stream, e.g.,a plurality of derivative streams. At least one of the derivativestreams, e.g., at least two or at least three of the derivative streams,comprises a second amount of the residual carbon monoxide. The secondamount of carbon monoxide is separate and different from the firstamount of residual carbon monoxide that is in the off-gas stream.Preferably, the at least one of the derivative streams is a vapor.Residual carbon monoxide includes carbon monoxide that has not reactedin the carbonylation reaction and, as such, remains in the crude aceticacid product and/or in the off-gas. In one embodiment, the residualcarbon monoxide in the derivative stream(s) is entrained in therespective stream. Without being bound by theory, it is believed thatthe entrainment of the carbon monoxide is due to the carbon monoxidebeing conveyed through the liquid reaction mixture. In contrast, thetypical off-gas stream is a stream that is removed from the vapor builtup in the reactor. Thus, the carbon monoxide in conventional off-gasstreams typically is not entrained in the stream. In one embodiment, thederivative stream(s) comprise less carbon monoxide than the carbonmonoxide fed to the primary reactor. In one embodiment, the derivativestream(s) comprise less than 95 mol % carbon monoxide, e.g., less than80 mol %, less than 75 mol %, less than 60 mol %, less than 50 mol %, orless than 40 mol %. In another embodiment, the vapor phase derivativestream(s) have a carbon monoxide partial pressure of less than 95% ofthe total pressure of the vapor phase derivative stream(s), e.g., lessthan 75%, less than 60%, less than 50%, or less than 40%. In terms ofranges, the derivative stream(s) optionally comprise from 10 mol % to 95mol % residual carbon monoxide, e.g., from 25 mol % to 75 mol %, or from40 mol % to 60 mol %. Preferably, the derivative stream(s) comprise from60 mol % to 70 mol % carbon monoxide. In terms of partial pressures, thederivative stream(s) optionally have a carbon monoxide partial pressureof from 10% to 95% of the total pressure of the derivative stream(s),e.g., from 25% to 75% or from 40% to 60%. In one embodiment, thederivative streams further comprise methanol and/or methanolderivative(s). For example, the derivative stream(s) may furthercomprise methanol and/or methanol derivatives in an amount less than 50mol %, e.g., less than 40 mol %, less than 25 mol % or less than 15 mol%. In terms of ranges, the derivative stream(s) may comprise from 5 mol% to 90 mol % methanol and/or methanol derivative, e.g., from 10 mol %to 60 mol %, or from 15 mol % to 30 mol %.

In addition, the process comprises the step of reacting, in a secondreactor, (i) at least a portion of the second amount of residual carbonmonoxide; and (ii) at least one of methanol and a methanol derivative,e.g., methyl acetate or dimethyl ether, to produce additional aceticacid. The reaction in the second reactor is preferably performed in afixed bed reactor or a trickle bed reactor. These reactors preferablycomprise a catalyst, e.g., a solid phase metal catalyst. The secondreactor provides for reaction of residual carbon monoxide, whichimproves overall process efficiency. As a result of the inventivecombination of steps, the inventive process achieves an overall carbonmonoxide conversion of at least 95%, e.g., at least 98%, at least 99% orat least 99.5%, based on the total amount of carbon monoxide in thecarbon monoxide feed. In a preferred embodiment, when utilizing theinventive secondary reactor, the overall carbon monoxide conversionachieved by the inventive processes (using a low purity carbon monoxidefeed) is at least equivalent to the overall carbon monoxide conversionsachieved in conventional processes, which employ a high purity carbonmonoxide feed stream, but do not utilize a secondary reaction. Inpreferred embodiments, the overall carbon monoxide conversion achievedby the inventive processes (using a low purity carbon monoxide feed) isat least at least 1% greater than the overall carbon monoxideconversions achieved in conventional processes, which employ a highpurity carbon monoxide feed stream, e.g., at least 5% greater than, atleast 10% greater than, or at least 25% greater than.

In one embodiment, at least a portion of the carbon monoxide in thepurge stream is reacted with at least one of methanol and a methanolderivative in the second reactor under conditions effective to produceadditional acetic acid. Preferably, at least a portion of the carbonmonoxide in the purge stream and at least a portion of the carbonmonoxide in the derivative stream are directed, optionally combined thendirected, to a secondary reactor and reacted with at least one ofmethanol and a methanol derivative, e.g., methyl acetate or dimethylether, to produce additional acetic acid. In such embodiments, theamount of total residual carbon monoxide that is converted to additionalacetic acid is greater than the amount of total residual carbon monoxidethat may be converted in conventional processes that do not convertresidual carbon monoxide in both the purge stream and the derivativestream(s), e.g., at least 1% greater, at least 5%, at least 10% greateror at least 25% greater. Accordingly, in some embodiments, less than 10mol %, e.g., less than 5 mol % or less than 1 mol % of the first andsecond amounts of residual carbon monoxide, in total, are vented fromthe process.

In another embodiment, the invention relates to a process for producingacetic acid comprising the step of contacting with at least one ofmethanol and a methanol derivative a secondary carbon monoxide feedstream, e.g., a secondary low carbon monoxide content feed stream,comprising a low concentration of carbon monoxide (as compared toconventional carbon monoxide feed streams), e.g., from 10 mol % to 95mol %, from 25 mol % to 75 mol % or from 40 mol % to 60 mol % carbonmonoxide. In one embodiment, the secondary carbon monoxide feed streamis the reaction product of a previous carbonylation reaction or aderivative of such a reaction product, which may have been separated,for example, to remove condensable liquids. Preferably, the low carbonmonoxide content feed stream comprises from 60 mol % to about 70 mol %carbon monoxide. In terms of partial pressures, the low carbon monoxidecontent feed streams optionally have a carbon monoxide partial pressureof from 10% to 95% of the total pressure of the derivative stream(s),e.g., from 25% to 75% or from 40% to 60%. In terms of limits, the lowcarbon monoxide feed stream preferably comprises less than 95 mol %carbon monoxide, e.g., less than 80 mol %, less than 70 mol %, less than50 mol %, or less than 40 mol %. In other embodiments, the low carbonmonoxide feed stream has a carbon monoxide partial pressure less than95% of the total pressure of the low carbon monoxide feed stream, e.g.,less than 80%, less than 70%, less than 50%, or less than 40%. Again,for carbon monoxide to be reacted with methanol to form acetic acid,some amount of carbon monoxide should be present in the carbon monoxidefeed stream. For example, the carbon monoxide feed stream may compriseresidual carbon monoxide in an amount greater than 0.1 mol %, greaterthan 0.5 mol % or greater than 1 mol %; or the carbon monoxide feedstream may have a carbon monoxide partial pressure greater than 0.1% ofthe total pressure of the low carbon monoxide feed stream, e.g., greaterthan 0.5% or greater than 1%. The carbon monoxide feed stream mayfurther comprise, for example, methanol and/or a methanol derivative,e.g., methyl acetate or dimethyl ether, which is used to produce anacetic acid composition. The inventive process reacts low carbonmonoxide feed streams, optionally obtained from vented gases, and thusavoids the waste thereof. Preferably, the contacting step is performedin a fixed bed reactor or a trickle bed reactor and over a catalyst,e.g., a solid phase metal catalyst, which may be fixed in a catalystbed. The catalyst may be specifically selected for use with the lowconcentration carbon monoxide feed stream.

In another embodiment, the invention is to a process for producingacetic acid comprising the step of contacting a secondary carbonmonoxide feed stream having a carbon monoxide partial pressure of lessthan 95% of the total pressure the carbon monoxide feed stream, e.g.,less than 90%, less than 80%, less than 70%, less than 50%, or less than40%, with at least one of methanol and a methanol derivative to producean acetic acid composition. Thus, this embodiment of the inventionutilizes a secondary feed stream having a partial pressure of carbonmonoxide lower than conventional processes, e.g., at least 5% lower, atleast 10% lower, at least 20% lower, or at least 50% lower. In oneembodiment, the secondary carbon monoxide feed stream is the reactionproduct of a previous carbonylation reaction or a derivative of such areaction product, which may have been separated, for example, to removecondensable liquids. Preferably, the contacting is performed over asolid phase metal catalyst. As noted above, for carbon monoxide to bereacted with methanol to form acetic acid, some amount of carbonmonoxide should be present in the carbon monoxide feed stream.Advantageously, this process has the capability to utilize carbonmonoxide streams that are less pure than conventional carbon monoxidefeed streams.

Also, an additional embodiment relates to a process for producing aceticacid comprising the step of contacting a carbon monoxide feed and atleast one of methanol and a methanol derivative under conditionseffective to produce a reaction product comprising a crude acetic acidproduct and a separation vent stream. The separation vent stream, in oneembodiment, results from the separation of the crude acetic acidproduct. The separation vent stream comprises low amounts of carbonmonoxide, e.g., less than 60 mol %, e.g., less than 50 mol %, less than25 mol %; less than 10 mol %; less than 5 mol %; or less than 1 mol %.In terms of ranges, the separation vent stream may comprise from 0.1 mol% to 60 mol % carbon monoxide, e.g., from 1 mol % to 50 mol %, or from 5mol % to 25 mol %. In another embodiment, the separation vent stream hasa carbon monoxide partial pressure less than 60% of the total pressureof the vent stream, e.g., less than 50%, less than 25%, less than 10%,less than 5%, or less than 1%. As such, less carbon monoxide is wastedthrough venting in the purification train and the overall carbonmonoxide conversion may be advantageously improved. In one embodiment,the overall carbon monoxide conversion relates to the conversion of theinitial carbon monoxide feed stream in a first reaction and theconversion of residual carbon monoxide in a second reaction. Overallcarbon monoxide conversions, are preferably greater than 90%, e.g.,greater than 95%, greater than 99%, or greater than 99.5%.

Carbonylation

The features of the present invention may be applied to any suitablemethanol carbonylation process. The formation of acetic acid via acarbon monoxide/methanol carbonylation reaction may be carried out byreacting methanol and/or methanol derivatives with carbon monoxide.Exemplary carbonylation systems 100 are shown in FIGS. 1-5.Carbonylation systems 100 comprise carbonylation reaction zone 101,separation zone 102 and a secondary reaction zone 103. Other exemplarycarbonylation systems, including reaction zone and separation zones,that may be used with embodiments of the present invention include thosedescribed in U.S. Pat. Nos. 7,223,886, 7,005,541, 6,6657,078, 6,339,171,5,731,252, 5,144,068, 5,026,908, 5,001,259, 4,994,608, and U.S. Pub.Nos. 2008/0287706, 2008/0293966, 2009/0107833, 2009/0270651, the entirecontents and disclosures of which are hereby incorporated by reference.Exemplary reaction zone 101 and separation zone 102 are shown in thedetailed schematic diagrams discussed below with respect to FIGS. 3-5.

As shown in FIGS. 1-5, methanol feed stream 104 and carbon monoxide feedstream 105 are fed, preferably continuously fed, to reaction zone 101 toproduce a crude acetic acid product 106. Crude acetic acid product 106may be fed to the separation zone 102 which produces a purified aceticacid product 107 and derivative streams 108 and 109, and optionalderivative stream 110. Derivative stream 108 may be fed to secondaryreaction zone 103. Derivative stream 109, which may comprise recycledcompounds, may be fed to reaction zone 101. In optional embodiments,optional derivative stream 110 may also be fed to secondary reactionzone 103. In one embodiment, the derivative stream(s) are streams thatare derived from the crude acetic acid product. For example, thederivative stream(s) may be streams that result from the separation ofthe crude acetic acid product. As another example, the derivativestreams may be stream(s) yielded by flasher 131. In one embodiment, thederivative stream(s) do not include conventional off-gas streams. Asnoted above, conventional off-gas streams are streams of by-productgases that build up in the reactor as the crude acetic acid product iswithdrawn, e.g., withdrawn into a flasher. These off-gas streamsessentially comprise reaction by-products and carbon monoxide, and donot result from the separation of the crude acetic acid product. Thus,conventional off-gas streams are not considered to be derivativestreams.

Because the present invention provides for efficient utilization ofresidual carbon monoxide, e.g., the residual carbon monoxide in thederivative stream(s), the inventive processes and systems allow largeramounts of off-gas to be withdrawn from the reactor and/or the flasher.These larger amounts of off-gas may beneficially be used to supplementother streams throughout the carbonylation/separation process, e.g., forcatalyst stabilization, for recycle streams, or for pump-around streams.In conventional systems, without secondary conversion of residual carbonmonoxide in derivative stream(s), larger amounts of off-gas purged wouldmerely result in increased waste of carbon monoxide, which would loweroverall carbon monoxide efficiencies. The utilization of the residualcarbon monoxide in the derivative stream(s) allows a larger off-gaspurge while still maintaining acceptable overall carbon monoxideefficiencies, e.g., without lowering overall carbon monoxideefficiencies.

Derivative stream 108 may be in the liquid-phase or vapor-phase andpreferably comprises dissolved and/or entrained residual carbon monoxideand optionally methanol and/or its reactive derivatives, preferablymethyl acetate. In preferred embodiments, derivative stream 108 is inthe vapor-phase. In FIG. 1, derivative stream 108 is fed to secondaryreactor 111, which preferably is a high pressure liquid phasecarbonylation secondary reactor. In FIG. 2, derivative stream 108 is fedto secondary reactor 112, which preferably is a low pressure gas phasecarbonylation secondary reactor.

The process conditions for the supplemental carbonylation reaction inthe secondary reactors 111, 112 may vary widely. The reaction may beconducted over a homogeneous or heterogeneous catalyst, e.g. a solidphase metal catalyst. The secondary carbonylation reaction may be ahomogeneous reaction or a heterogeneous reaction. In one embodiment, thecatalyst may be similar to the catalyst used for the carbonylationreaction in reaction zone 101, discussed further below. In anotherembodiment, the catalyst may be a liquid phase catalyst. Also, thereaction in the second reactor (and optionally that in the firstreactor, as well) may be conducted in a counter-current or co-currentmanner, with a vapor phase co-current reaction being preferred. Althoughthe catalyst for the reaction in the secondary reactors 111, 112 may bethe same as the catalyst in the reaction zone 101, it is preferred thatthe catalyst in the secondary reactors 111, 112 are different from thecatalyst in the reaction zone 101. Preferably, the catalyst in thesecondary reactors 111, 112 is tailored to account for a carbon monoxidestream that comprises lower amounts of carbon monoxide. Preferably, thecatalyst in the second reactor is a rhodium diiodide dicarbonyl anionthat is ionically bound to a suitable resin, e.g., polyvinylpyridine orcarbon.

The secondary reactors 111, 112, generally, may be any reactor suitablefor carbonylation of methanol with a relatively low carbon monoxide feedstream. In preferred embodiments, the secondary reactors 111, 112 areeach independently a trickle bed reactor and/or a fixed bed reactor.Trickle bed reactors and fixed bed reactors preferably comprise a solidphase metal catalyst fixed or packed in a catalyst bed. In oneembodiment, each of the secondary reactors 111, 112 may comprisecatalyst section 113 and a head space 114.

Derivative stream 108 fed to the secondary reaction zone 103 preferablycomprises a relatively lower concentration of carbon monoxide than isfed to the carbonylation reaction zone 101. In one embodiment, theconcentration of carbon monoxide in derivative stream 108 may be atleast 5% lower than the concentration of the carbon monoxide fed to thereaction zone 101, e.g., at least 10% lower, at least 25% lower, or atleast 50% lower. In other embodiments, the carbon monoxide concentrationof derivative stream 108 (either in mol % or carbon monoxide partialpressure) may be at least 5% lower than the concentration in aconventional off-gas stream, e.g., at least 10% lower, at least 25%lower, or at least 50% lower. In one embodiment, because of therelatively low amount of carbon monoxide in the feed, the molar ratio ofother reactants, e.g., methanol and/or methanol derivatives, to carbonmonoxide in the second reactor is greater than 0.02:1, e.g., greaterthan 0.1:1, greater than 0.25:1 or greater than 0.5:1.

In preferred embodiments, the reactant, e.g., methanol and/or methanolderivatives, reacted in the secondary reactors 111, 112, may be presentin derivative stream 108. In one embodiment, derivative stream 108comprises methanol and/or methanol derivative in an amount ranging from5 mol % to 90 mol %, e.g., from 25 mol % to 75 mol % or from 40 mol % to60 mol %. In other embodiments, derivative stream 108 is in the vaporphase and has a methanol and/or methanol derivative partial pressure offrom 10% to 90% of the total pressure of derivative stream 108, e.g.,from 25% to 75% of from 40% to 60%. In preferred embodiments themethanol and/or methanol derivative reactant in secondary reactors 111,112 is methyl acetate. In optional embodiments, fresh methanol and/ormethanol derivatives may be fed to secondary reactors 111, 112, via line115. In other optional embodiments, methanol and/or methanol derivativescontained in the optional derivative stream 110 from the separation zone102 may be fed to secondary reactors 111, 112, as shown in FIGS. 1 and2, respectively. In other embodiments, derivative stream 108 comprisesacetaldehyde. In these embodiments, secondary reactor 111 may react theacetaldehyde in derivative stream 108 to form other materials. Forexample, the acetaldehyde may be reacted to form ethanol, which may thenbe converted to propionic acid, which is easily removed from the productstream. By converting the acetaldehyde in derivative stream 108,acetaldehyde is advantageously removed from derivative stream 108. Thisreaction of acetaldehyde lowers the amount of acetaldehyde in theproduct stream and lessens the need for subsequent acetaldehyde removalunits, e.g., PRS units.

In FIG. 1, secondary reactor 111 preferably is a high pressure liquidphase carbonylation secondary reactor. The supplemental carbonylationreaction in the secondary reactor 111 may be conducted over aliquid-phase homogeneous catalyst or a solid heterogeneous catalyst. Inone embodiment, the liquid-phase homogeneous catalyst comprises metaldissolved in a solution, e.g., rhodium and/or iridium dissolved inacetic acid. In one embodiment, the reaction in secondary reactor 111 iscarried out at a pressure of from 0.1 MPa to 10 MPa, e.g., from 1 MPa to5 MPa or from 2 MPa to 3 MPa and a temperature of from 100° C. to 350°C., e.g., 150° C. to 300° C. or 175° C. to 250° C. Secondary reactor 111is preferably operated at lower pressure than that of the primaryreactor. In one embodiment, secondary reactor 111 is operated at atemperature similar to that of the primary reactor. In other embodiment,secondary reactor 11 is operated at a temperature that is higher, e.g.,at least 5% higher or at least 10% higher, than the temperature of theprimary reactor.

Derivative stream 108 is preferably fed to secondary reactor 111 as aliquid or as a condensed vapor stream, optionally with fresh reactants115, to produce a secondary crude product stream 116 that comprisesacetic acid and overhead stream 117. Overhead stream 117 comprisesmethyl iodide, residual carbon monoxide, vaporized methanol, vaporizedmethyl acetate, and other non-condensable gases such as methane.Overhead stream 117 is condensed and fed to a knock-out pot 118 toremove a liquid stream 119 and a vapor stream 120. Liquid stream 119,along with optional derivative stream 110, is sprayed on the catalystsection 114 in secondary reactor 111.

Secondary crude product stream 116 may be processed further and fed tothe separation zone 102 or may be combined with the purified acetic acidproduct 107. In some embodiments, secondary crude product stream 116 maybe recovered independently of purified acetic acid product 107.Preferably, the secondary crude product stream 116 is enriched in aceticacid relative to derivative stream 108. In one embodiment, secondarycrude product stream 116 comprises from 30 mol % to 95 mol % aceticacid, e.g., from 50 mol % to 75 mol % or from 45 mol % to 70 mol %. Interms of limits, secondary crude product stream 116 comprises at least25 mol % acetic acid, e.g., at least 50 mol %, at least 40 mol % or atleast 60 mol %. In terms of partial pressures, secondary crude productstream 116 (when in the vapor phase) may have an acetic acid partialpressure of from 30% to 95% of the total pressure of secondary crudeproduct stream 116, e.g., from 50% to 75% or from 45% to 70%. In oneembodiment, secondary crude product stream 116 may further comprise lowamounts of carbon monoxide, e.g., less than 40 mol % carbon monoxide,e.g., less than 25 mol %, less than 10 mol %, less than 5 mol %, or lessthan 3 mol %. In other embodiments, secondary crude product stream 116may further comprise methanol and/or methanol derivatives in an amountless than 50 mol %, e.g., less than 40 mol %, less than 25 mol % or lessthan 15 mol %. In terms of ranges secondary crude product stream 116 maycomprise from 10 mol % to 50 mol % methanol and/or methanol derivative,e.g., from 10 mol % to 40 mol %, or from 15 mol % to 30 mol %.

Vapor stream 120 may be purged or flared as shown. In preferredembodiments, vapor stream 120 comprises substantially less carbonmonoxide, and more preferably essentially no carbon monoxide, thanderivative stream 108. In addition, a portion of vapor stream 120 may befed to one or more recovery unit 121. As shown in FIG. 1, there isprovided one recovery unit 121. A scrubbing solvent 122, preferablychilled to less than 25° C., may be fed to recovery unit 121 to scrubvapor stream 120 of low boiling point components, such as methyl iodide,which are removed via line 123 and are preferably returned to thereaction zone 101. Exemplary scrubbing solvents include methanol, methylacetate, dimethyl ether, acetic acid and mixtures thereof. The overheadsof recovery unit 121 may exited as purge gas 124.

In FIG. 2, secondary reactor 112 preferably is a low pressure gas phasecarbonylation secondary reactor. The supplemental carbonylation reactionin the secondary reactor 112 may be reacted with a heterogeneouscatalyst. In one embodiment, the reaction in the secondary reactor iscarried out at a pressure of from 0.01 MPa to 10 MPa, e.g., 0.05 MPa to5 MPa or 0.05 MPa to 1 MPa and a temperature of from 150° C. to 350° C.,e.g., 150° C. to 300° C. or 175° C. to 250° C. By conducting thereaction in the secondary reactor at lower pressures, the burden onsystem components, e.g., pumps and compressors, may be reduced. Also,because lower temperature and/or lower pressure operation is lesscorrosive, vessels need not be made from expensive corrosion-resistantmetals and less-expensive metals, e.g., standard stainless steel, may beused.

The derivative stream 108 is preferably fed to secondary reactor 112 asa vapor, optionally with derivative stream 110, to produce a secondarycrude product stream 125. In optional embodiment, fresh reactants may beadded to head space 114 of the secondary reactor 112. The secondarycrude product stream 125 is condensed and fed to knock-out pot 126 toremove a liquid stream 127 comprising acetic acid and a vapor stream128. Liquid stream 127 may be processed further and fed to theseparation zone 102 or may be combined with the purified acetic acidproduct 107. In some embodiments, secondary crude product stream 125 maybe recovered independently of purified acetic acid product 107.Preferably, liquid stream 127 is enriched in acetic acid relativederivative stream 108. In one embodiment liquid stream 127 comprisesfrom 30 mol % to 95 mol % acetic acid, e.g., from 50 mol % to 75 mol %or from 45 mol % to 70 mol %. In terms of limits, stream 127 comprisesat least 25 mol % acetic acid, e.g., at least 50 mol %, at least 40 mol% or at least 60 mol %. In one embodiment, stream 127 may furthercomprise low amounts of carbon monoxide, e.g., less than 40 mol % carbonmonoxide, less than 25 mol %, less than 10 mol %, less than 5 mol %, orless than 3 mol %. In other embodiments, stream 127 may further comprisemethanol and/or methanol derivatives in an amount less than 50 mol %,e.g., less than 40 mol %, less than 25 mol % or less than 15 mol %. Interms of ranges, stream 127 may comprise from 10 mol % to 50 mol %methanol and/or methanol derivative, e.g., from 10 mol % to 40 mol %, orfrom 15 mol % to 30 mol %.

In some embodiments, where the secondary reaction is conducted in thevapor phase, the reaction temperature may be maintained at a temperaturebelow the dew point of acetic acid. In such cases, the resultant aceticacid product will contain an amount of rhodium. This amount of rhodiummay be greater than the amount of rhodium, if any, in conventionalacetic acid products that do not use the inventive processes and/orsystems.

Vapor stream 128 in FIG. 2 may be purged or flared as shown. Inpreferred embodiments, vapor stream 128 comprises substantially lesscarbon monoxide, and more preferably comprises essentially no carbonmonoxide, than derivative stream 108. In addition, a portion of vaporstream 128 may be fed to one or more recovery unit 121, as discussedabove with reference to FIG. 1.

Returning to the reaction zone, an exemplary reaction zone 101 is shownin FIGS. 3-5. Reaction zone 101 comprises a first reactor 130, flasher131 and a reactor recovery unit 132. In some embodiments of the presentinvention, the primary carbonylation reaction is conducted in firstreactor 130. Low purity carbon monoxide feed stream 105 has thecharacteristics discussed above. In one embodiment, the carbonylation isachieved by reacting carbon monoxide with methanol in first reactor 130,e.g., a continuous stirred tank reactor (“CSTR”). When using a CSTR, thecatalyst is dissolved in the reaction solvent and liquid methanol andcarbon monoxide gas are injected from the bottom as reaction rawmaterials and made to react with one another. When a CSTR is utilized,the CSTR may be adapted to agitate the reaction solution by an agitationdevice such as an impeller. Alternatively, a bubble column reactor maybe utilized as the first reactor to perform the carbonylation. When abubble column reactor is utilized, a cylindrical reactor is filled witha reaction solvent and a solid catalyst. Liquid methanol is suppliedfrom the bottom as reaction raw material while carbon monoxide gas isinjected upward from the bottom as jet stream. The injected carbonmonoxide gas forms bubbles as it rises in the liquid contained in thecylindrical reactor and particles of the catalyst are also driven tomove upward in the cylindrical reactor by the gas lift effect anddispersed into the liquid. As one example, the carbon monoxide may beinjected into the liquid contained in a cylindrical reactor as jetstream by way of a nozzle arranged at the bottom of the cylindricalreactor for the purpose of mobilizing particles of the solid catalyst inthe reactor, as disclosed in Japanese Patent Application Laid-Open No.6-340242, the disclosure of which is hereby incorporated by reference.In another embodiment, a plurality of primary reactors may be used andthe various configurations of primary reactors are considered to bewithin the scope of the present invention. Preferably, the carbonylationprocess is a low water, catalyzed, e.g., rhodium-catalyzed,carbonylation of methanol to acetic acid, as exemplified in U.S. Pat.No. 5,001,259, which is hereby incorporated by reference in itsentirety.

The present invention may be appreciated in connection with, forexample, the carbonylation of methanol with carbon monoxide in ahomogeneous catalytic reaction system comprising a reaction solvent,methanol and/or reactive derivatives thereof, a Group VIII catalyst, atleast a finite concentration of water, and optionally an iodide salt.

Suitable Group VIII catalysts include rhodium and/or iridium catalysts.When a rhodium catalyst is utilized, the rhodium catalyst may be addedin any suitable form such that the active rhodium catalyst is a carbonyliodide complex. Exemplary rhodium catalysts are described in MichaelGauβ, et al., Applied Homogeneous Catalysis with OrganometallicCompounds: A Comprehensive Handbook in Two Volume, Chapter 2.1, p.27-200, (1^(st) ed., 1996). Iodide salts optionally maintained in thereaction mixtures of the processes described herein may be in the formof a soluble salt of an alkali metal or alkaline earth metal or aquaternary ammonium or phosphonium salt. In certain embodiments, thecatalyst co-promoter is lithium iodide, lithium acetate, or mixturesthereof. The salt co-promoter may be added as a non-iodide salt thatwill generate an iodide salt. The iodide catalyst stabilizer may beintroduced directly into the reaction system. Alternatively, the iodidesalt may be generated in-situ since under the operating conditions ofthe reaction system, a wide range of non-iodide salt precursors willreact with methyl iodide or hydroiodic acid in the reaction medium togenerate the corresponding co-promoter iodide salt stabilizer. Foradditional detail regarding rhodium catalysis and iodide saltgeneration, see U.S. Pat. Nos. 5,001,259; 5,026,908; and 5,144,068, theentireties of which are hereby incorporated by reference.

When an iridium catalyst is utilized, the iridium catalyst may compriseany iridium-containing compound which is soluble in the liquid reactioncomposition. The iridium catalyst may be added to the liquid reactioncomposition for the carbonylation reaction in any suitable form whichdissolves in the liquid reaction composition or is convertible to asoluble form. Examples of suitable iridium-containing compounds whichmay be added to the liquid reaction composition include: IrCl₃, IrI₃,IrBr₃, [Ir(CO)₂I]₂, [Ir(CO)₂Cl]₂, [Ir(CO)₂Br]₂, [Ir(CO)₂I₂]⁻H⁺,[Ir(CO)₂Br₂]⁻H⁺, [Ir(CO)₂I₄]⁻H⁺, [Ir(CH₃)I₃(CO₂]⁻H⁺, Ir₄(CO)₁₂,IrCl₃.3H₂O, IrBr₃.3H₂O, Ir₄(CO)₁₂, iridium metal, Ir₂O₃, Ir(acac)(CO)₂,Ir(acac)₃, iridium acetate, [Ir₃O(OAc)₆(H₂O)₃][OAc], andhexachloroiridic acid [H₂IrCl₆]. Chloride-free complexes of iridium suchas acetates, oxalates and acetoacetates are usually employed as startingmaterials. The iridium catalyst concentration in the liquid reactioncomposition may be in the range of 100 to 6000 ppm. The carbonylation ofmethanol utilizing iridium catalyst is well known and is generallydescribed in U.S. Pat. Nos. 5,942,460; 5,932,764; 5,883,295; 5,877,348;5,877,347 and 5,696,284, the entireties of which are hereby incorporatedby reference.

An alkyl halide co-catalyst/promoter is generally used in combinationwith the Group VIII metal catalyst component. Methyl iodide is preferredas the alkyl halide promoter. Preferably, the concentration of alkylhalide in the liquid reaction composition is in the range of 1 to 50% byweight, preferably 15 to 25% by weight.

The halogen promoter may be combined with a salt stabilizer/co-promotercompound, which may include salts of a metal of Group IA or Group IIA, aquaternary ammonium, phosphonium salt or mixtures thereof. Particularlypreferred are iodide or acetate salts, e.g., lithium iodide or lithiumacetate.

Other promoters and co-promoters may be used as part of the catalyticsystem of the present invention as described in U.S. Pat. No. 5,877,348,the entirety of which is hereby incorporated by reference. Suitablepromoters are selected from ruthenium, osmium, tungsten, rhenium, zinc,cadmium, indium, gallium, mercury, nickel, platinum, vanadium, titanium,copper, aluminum, tin, antimony, and are more preferably selected fromruthenium and osmium. Specific co-promoters are described in U.S. Pat.No. 6,627,770, the entirety of which is incorporated herein byreference.

A promoter may be present in an effective amount up to the limit of itssolubility in the liquid reaction composition and/or any liquid processstreams recycled to the carbonylation reactor from the acetic acidrecovery stage. When used, the promoter is suitably present in theliquid reaction composition at a molar ratio of promoter to metalcatalyst of 0.5:1 to 15:1, preferably 2:1 to 10:1, more preferably 2:1to 7.5:1. A suitable promoter concentration is 400 to 5000 ppm.

In one embodiment, the temperature of the carbonylation reaction in thefirst reactor is preferably from 150° C. to 250° C., e.g., from 150° C.to 225° C., or from 150° C. to 200° C. The pressure of the carbonylationreaction is preferably from 1 to 20 MPa, preferably 1 to 10 MPa, mostpreferably 1.5 to 5 MPa Acetic acid is typically manufactured in aliquid phase reaction at a temperature of from about 150-200° C. and atotal pressure from about 2 to about 5 MPa.

In one embodiment, reaction mixture comprises a reaction solvent ormixture of solvents. The solvent is preferably compatible with thecatalyst system and may include pure alcohols, mixtures of an alcoholfeedstock, and/or the desired carboxylic acid and/or esters of these twocompounds. In one embodiment, the solvent and liquid reaction medium forthe (low water) carbonylation process is preferably acetic acid.

Methanol feed stream 104 preferably comprises methanol and/or reactivederivatives thereof. Suitable reactive derivatives of methanol includemethyl acetate, dimethyl ether, and methyl formate. In one embodiment, amixture of methanol and reactive derivatives thereof may be used asreactants in the process of the present invention. Preferably, methanoland/or methyl acetate are used as reactants. At least some of themethanol and/or reactive derivative thereof will be converted to, andhence present as, methyl acetate in the liquid reaction composition byreaction with acetic acid product or solvent. The concentration in theliquid reaction composition of methyl acetate is suitably in the rangeof from 0.5 wt. % to 70 wt. %, e.g., from 0.5 wt. % to 50 wt. %, from 1wt. % to 35 wt. %, or from 1 wt. % to 20 wt. %.

Carbon monoxide feed stream 105 is a lower purity carbon monoxide feedstream as described above. Hydrogen may be generated in the carbonmonoxide feed stream by the water gas shift reaction. Preferably, thepartial pressure of hydrogen is maintained at a low level, for example,less than 0.1 MPa or less than 0.05 MPa, as its presence may result inthe formation of various hydrogenation products. In one embodiment, thepartial pressure of carbon monoxide in the reaction is in the range offrom 0.1 MPa to 7 MPa, e.g., from 0.1 MPa to 3.5 MPa, or from 0.1 MPa to1.5 MPa.

Water may be formed in situ in the liquid reaction composition, forexample, by the esterification reaction between methanol reactant andacetic acid product. Water may be introduced to the carbonylationreactor together with or separately from other components of the liquidreaction composition. Water may be separated from other components ofreaction composition withdrawn from the reactor and may be recycled incontrolled amounts to maintain the required concentration of water inthe liquid reaction composition. Preferably, the concentration of watermaintained in the liquid reaction composition is in the range of from0.1 wt. % to 16 wt. %, e.g., from 1 wt. % to 14 wt. %, or from 1 wt. %to 10 wt. %.

In accordance with a preferred carbonylation process according to thepresent invention, the desired reaction rates are obtained even at lowwater concentrations by maintaining in the reaction medium an ester ofthe desired carboxylic acid and an alcohol, desirably the alcohol usedin the carbonylation, and an additional iodide ion that is over andabove the iodide ion that is present as hydrogen iodide. An example of apreferred ester is methyl acetate. The additional iodide ion isdesirably an iodide salt, with lithium iodide (LiI) being preferred. Ithas been found, as described in U.S. Pat. No. 5,001,259, that under lowwater concentrations, methyl acetate and lithium iodide act as ratepromoters only when relatively high concentrations of each of thesecomponents are present and that the promotion is higher when both ofthese components are present simultaneously. The concentration of iodideion maintained in the reaction medium of the preferred carbonylationreaction system is believed to be quite high as compared with whatlittle prior art there is dealing with the use of halide salts inreaction systems of this sort. The absolute concentration of iodide ioncontent is not a limitation on the usefulness of the present invention.

The carbonylation reaction of methanol to acetic acid product may becarried out by contacting the methanol feed with gaseous carbon monoxidebubbled through an acetic acid solvent reaction medium containing thecatalyst e.g., rhodium or iridium, methyl iodide promoter, methylacetate, and/or additional soluble iodide salt, at conditions oftemperature and pressure suitable to form the carbonylation product. Itwill be generally recognized that it is the concentration of iodide ionin the catalyst system that is important and not the cation associatedwith the iodide, and that at a given molar concentration of iodide thenature of the cation is not as significant as the effect of the iodideconcentration. Any metal iodide salt, or any iodide salt of any organiccation, or other cations such as those based on amine or phosphinecompounds (optionally, ternary or quaternary cations), can be maintainedin the reaction medium provided that the salt is sufficiently soluble inthe reaction medium to provide the desired level of the iodide. When theiodide is a metal salt, preferably it is an iodide salt of a member ofthe group consisting of the metals of Group IA and Group IIA of theperiodic table as set forth in the “Handbook of Chemistry and Physics”published by CRC Press, Cleveland, Ohio, 2002-03 (83rd edition). Inparticular, alkali metal iodides are useful, with lithium iodide beingparticularly suitable.

In low water carbonylation, the additional iodide over and above theorganic iodide promoter may be present in the catalyst solution inamounts ranging from 2 wt. % to 20 wt. %, e.g., from 2 wt. % to 15 wt.%, or from 3 wt. % to 10 wt. %; the methyl acetate may be present inamounts ranging from 0.5 wt % to 30 wt. %, e.g., from 1 wt. % to 25 wt.%, or from 2 wt. % to 20 wt. %; and the lithium iodide may be present inamounts ranging from 5 wt. % to 20 wt %, e.g., from 5 wt. % to 15 wt. %,or from 5 wt. % to 10 wt. %. The catalyst may be present in the catalystsolution in amounts ranging from 200 wppm to 2000 wppm, e.g., from 200wppm to 1500 wppm, or from 500 wppm to 1500 wppm.

First reactor 130 is preferably either a stirred vessel, e.g., CSTR, orbubble-column type vessel, with or without an agitator, within which thereaction medium is maintained, preferably automatically, at apredetermined level. This predetermined level may remain substantiallyconstant during normal operation. Into first reactor 130, methanol,carbon monoxide, and sufficient water may be continuously introduced asneeded to maintain at least a finite concentration of water in thereaction medium.

In one embodiment, carbon monoxide, e.g., in the gaseous state, iscontinuously introduced into first reactor 130, desirably below theagitator, which is used to stir the contents. The temperature of firstreactor 130 may be controlled, as indicated above. Carbon monoxide feed105 is introduced at a rate sufficient to maintain the desired totalreactor pressure.

The gaseous feed is preferably thoroughly dispersed through the reactionmedium by the stirring means. A carbon monoxide purge stream, e.g., anoff-gas, 132 is purged from first reactor 130. Purge stream 132 preventsbuildup of impurities remaining from the carbon monoxide feed, e.g.,inerts, and gaseous by-products, e.g., methane, carbon dioxide, andhydrogen, and also maintains a set carbon monoxide partial pressure at agiven total reactor pressure. Purge stream 132 is not a derivativestream, as discussed above. Purge stream 132, advantageously, is largerthan conventional purge streams, e.g., off-gas streams. In oneembodiment, purge stream 132 comprises a higher volume percentage ofcarbon monoxide, as compared to conventional purge streams, although, inother embodiments, the volume percentage of carbon monoxide in purgestream 132 is similar to those of conventional purge streams. As aresult of the larger purge streams, the inventive processes are able toprovide larger amounts of carbon monoxide for use throughout thecarbonylation process, e.g., for catalyst stabilization. These largeramounts of carbon monoxide are achieved without feeding additionalcarbon monoxide to the system and while still achieving overall carbonmonoxide efficiencies equal to or greater than those of conventionalcarbonylation systems.

As shown in FIGS. 3-5, reactor recovery unit 133 may be utilized toseparate impurities from the purge stream in line 132 to form animpurity stream (not shown) and a purified off-gas (not shown). In oneembodiment, reactor recovery unit removes low boiling point componentsfrom purge stream 132. Of course, other separation methods may beemployed to separate and/or purify purge stream 132. Purge stream(s) 132from first reactor 130 may be combined or scrubbed separately and aretypically scrubbed with either acetic acid, methanol, or mixtures ofacetic acid and methanol to prevent loss of low boiling components suchas methyl iodide from the process. If methanol is used as the purgescrub liquid solvent, the enriched methanol (containing methyl iodide)from reactor recovery unit 133 is typically returned to the process,e.g., via line 134, although it can also be returned into any of thestreams that recycle back to the reactor such as the flasher residue orlight ends or dehydration column overhead streams. If acetic acid isused as the purge scrub liquid solvent, the enriched acetic acid(containing methyl iodide) from the scrubber is typically stripped ofabsorbed light ends and the resulting lean acetic acid is recycled backto the scrubber (not shown). The light end components stripped from theenriched acetic acid scrubbing solvent may be returned to the mainprocess directly, e.g., via line 134, or indirectly in several differentlocations including the first reactor 130, flasher 131, e.g., via line135, or a suitable area in the separation zone 102. In one embodiment,the stream exiting the top of reactor recovery unit 133 is exited vialine 139 to further processing, which may entail, for example, furtherseparation or scrubbing. Preferably, the contents of line 136, which maycontain, inter alia, (residual) carbon monoxide and methanol, may befurther reacted, preferably in secondary reaction zone 103, to produceadditional acetic acid. Optionally, the gaseous purge streams may bevented through the flasher base liquid or lower part of the light endscolumn to enhance rhodium stability and/or they may be combined withother gaseous process vents (such as the purification column overheadreceiver vents) prior to scrubbing.

Purge stream 132 comprises an amount of residual carbon monoxide, e.g.,a first amount of residual carbon monoxide. In one embodiment, at leasta portion, e.g., an aliquot portion, of purge stream 132 is directed toa reactor, e.g., primary reactor 130, where the residual carbon monoxidemay be converted into acetic acid. In other embodiments, purge stream132 is directed to secondary reactor 113 where the residual carbonmonoxide may be converted into acetic acid. In other embodiments, purgestream 132 is directed to a reactor other than primary reactor 130 orsecondary reactor 113. In another embodiment, purge stream 132 may becombined with one or more derivative streams that comprise residualcarbon monoxide and then directed to primary reactor 130, secondaryreactor 113 and/or another reactor. In another embodiment, at least aportion, e.g., an aliquot portion, of purge stream 132 may be directedto separation zone 102, e.g., one or more of the components ofseparation zone 102, which is configured downstream of first reactor130. In these cases, as one example, the carbon monoxide in purge stream132 may be used to stabilize catalyst that may be present in the processstreams of separation zone 102. For example, at least a portion of purgestream 132 may be directed to light ends column 140. In anotherembodiment, at least a portion of purge stream 132 may be directed toflasher 131. In these cases, the at least a portion of purge stream 132may comprise at least 20 mol % carbon monoxide, e.g., at least 40 mol %or at least 60 mol % In one embodiment, the purge stream 132 isseparated into two or more carbon monoxide-containing off-gas streams,each of which may be directed to separation zone 102 and/or to thereactor and/or to flasher 131. In one embodiment, all of purge stream132 is directed to flasher 131. In one embodiment, all of purge stream132 is directed to separation zone 102, e.g., one or more of thecomponents of separation zone 102. In one embodiment, a portion of purgestream 132 may be directed to flasher 131 and another portion of purgestream 132 may be directed to secondary reactor 113 and/or anotherreactor.

The crude acetic acid product comprises an amount of unreacted carbonmonoxide, e.g., a second amount of residual carbon monoxide. As notedabove, in some embodiments, the acetic acid in the crude acetic acidproduct is separated into purified acetic acid and at least one, e.g.,at least two or at least three, derivative streams. In preferredembodiments, at least one, e.g., at least two or at least three, ofthese derivative streams comprises residual carbon monoxide. Thederivative stream comprising residual carbon monoxide may bebeneficially further reacted to form additional acetic acid.

The crude acetic acid product is drawn off from the first reactor 130 ata rate sufficient to maintain a constant level therein and is providedto flasher 131 via stream 136. In flasher 131, the crude product isseparated in a flash separation step to obtain a volatile (“vapor”)overhead stream 137 comprising acetic acid and a less volatile stream138 comprising a catalyst-containing solution. The catalyst-containingsolution comprises acetic acid containing the rhodium and the iodidesalt along with lesser quantities of methyl acetate, methyl iodide, andwater. The less volatile stream 138 preferably is recycled to reactor130. Vapor overhead stream 137 also comprises methyl iodide, methylacetate, water, PRCs. Dissolved and/or entrained gases exit firstreactor 130 and enter flasher 131 comprise a portion of the carbonmonoxide and may also contain gaseous by-products such as methane,hydrogen, and carbon dioxide. The dissolved gases exit the flasher 131as part of overhead stream 137. In some embodiments, the low-boilingoverhead vapor stream 137 may be fed to the secondary reaction zone 103,e.g., to the secondary reactor.

Overhead stream 137 from flasher 131 is directed to separation zone 102.Separation zone 102 comprises light ends column 140, decanter 141, anddrying column 142. Additionally, separation zone 102 may also compriseone or more columns for removing permanganate reducing compounds(“PRCs”), guard beds, heavy ends columns, extractors, etc.

In light ends column 140, stream 137 yields a low-boiling overhead vaporstream 143, a purified acetic acid product that preferably is removedvia a side stream 144, and a high boiling residue stream 145. Aceticacid removed via side stream 144 preferably is subjected to furtherpurification, such as in drying column 142 for selective separation ofacetic acid from water and/or an optional heavy ends column (not shown),as described in U.S. Pat. No. 6,627,770, the entirety of which is herebyincorporated by reference. Preferably, side stream 144 and residuestream 145 comprise substantially no carbon monoxide or no detectableamounts of carbon monoxide.

The low-boiling overhead vapor in line 143 may comprise dissolved and/orentrained carbon monoxide; methyl iodide; methyl acetate; hydrogen;water; PRCs; acetic acid; inerts such as nitrogen, argon, and helium;and other dissolved gases. In terms of upper limits, the low-boilingoverhead vapor in line 143 may comprise less than 75 mol % carbonmonoxide, e.g., less than 60 mol %; less than 50 mol %, or less than 40mol %; and/or may have a carbon monoxide partial pressure less than 75%of the total pressure of the low-boiling overhead vapor, e.g., less than60%, less than 50%, or less than 40%. Preferably, the amount ofdissolved and/or entrained carbon monoxide in line 143 preferably isless than the amount of carbon monoxide in feed stream 105, e.g., atleast 5% less, at least 10% less, at least 25% less, or at least 50%less. In terms of ranges, the amount of carbon monoxide in line 143 mayrange from 10 mol % to 75 mol %, e.g., from 25 mol % to 60 mol %, orfrom 40 mol % to 50 mol %; or the carbon monoxide partial pressure mayrange from 10% to 75% of the total pressure of low-boiling overheadvapor, e.g., from 25% to 60% of from 40% to 50%. Preferably, thelow-boiling overhead vapor in line 143 comprises at least 0.1 mol %carbon monoxide, e.g., at least 0.5 mol %, or at least 1; and/or has acarbon monoxide partial pressure of at least 0.1% of the total pressureof the low-boiling overhead vapor, e.g., at least 0.5% or at least 1%.Also, the low-boiling overhead vapor in stream 143 may comprise at least0.1 mol % methyl iodide, e.g., at least 1 mol %, or at least 5 mol %. Interms of ranges, stream 143 may comprise from 0.1 mol % to 30 mol %methyl iodide, e.g., from 1 mol % to 25 mol %, or from 5 mol % to 20 mol%. In some embodiments, the derivative stream of the crude acetic acidproduct in line 143 may be fed to the secondary reaction zone 103.

U.S. Pat. Nos. 6,143,930 and 6,339,171 discloses that there is generallya higher concentration of PRCs, and in particular acetaldehyde, in thelow-boiling overhead vapor stream 143 exiting the light ends column 140than in the high-boiling residue stream 145. In some embodiment,low-boiling overhead vapor stream 140, containing PRCs, optionally maybe subjected to additional processing in a PRC removal system (“PRS”)(not shown) to reduce and/or remove the amount of PRCs present (or aportion thereof). PRCs are formed during the carbonylation of methanolin the presence of a Group VIII metal carbonylation catalyst. PRCs, mayinclude, for example, compounds such as acetaldehyde, acetone, methylethyl ketone, butyraldehyde, crotonaldehyde, 2-ethyl crotonaldehyde,2-ethyl butyraldehyde and the like, and the aldol condensation productsthereof.

As shown, low-boiling overhead vapor stream 143, is preferably condensedand directed to an overhead phase separation unit, as shown by overheadreceiver decanter 141. Conditions are desirably maintained in theprocess such that low-boiling overhead vapor stream 143, once indecanter 141, will separate into a light phase and a heavy phase.Generally, low-boiling overhead vapor stream 143 is cooled to atemperature sufficient to condense and separate the condensable methyliodide, methyl acetate, acetaldehyde and other carbonyl components, andwater into two phases. A gaseous portion of stream 143 may includecarbon monoxide, and other noncondensable gases such as methyl iodide,carbon dioxide, hydrogen, and the like and is vented from the decanter141 via line 146. Line 146 preferably has a partial pressure of carbonmonoxide of less than 95% of the total pressure of line 146, e.g., lessthan 80%; less than 75%; less than 60%; less than 50%; or less than 40%.As used herein, all partial pressures are based upon the total pressureof all non-condensable components in the specified stream or vessel.Additionally or alternatively, line 146 may comprise less than 95 mol %carbon monoxide, e.g., less than 80 mol %; less than 75 mol %; less than60 mol %; less than 50 mol %; or less than 40 mol %. Line 146 preferablyhas a carbon monoxide partial pressure and/or a weight percentage thatis lower than carbon monoxide feed stream 105, which feeds first reactor130, e.g., 5% lower; 10% lower; 25% lower or 50% lower. In terms ofranges, the amount of carbon monoxide in line 146 optionally ranges from10 mol % to 95 mol %, e.g., from 25 mol % to 75 mol %, or from 40 mol %to 60 mol %. Optionally, line 146 has a carbon monoxide partial pressureof from 10% to 95% of the total pressure of line 146, e.g., from 25% to75% or from 40% to 60%. Line 146 preferably has a mole percentage ofmethyl acetate that optionally ranges from 10 mol % to 60 mol %, e.g.,from 15 mol % to 50 mol %, or from 25 mol % to 45 mol %; and/or a methylacetate partial pressure of from 10% to 60% of the total pressure ofline 146, e.g., from 15% to 50% or from 25% to 45%. This derivativestream in line 146 comprising carbon monoxide may be directed to asecondary reaction zone 103, for reaction with methanol to formadditional acetic acid, as discussed above in FIGS. 1 and 2.

The condensed light phase 147 in decanter 141 preferably compriseswater, acetic acid, and PRCs, as well as quantities of methyl iodide andmethyl acetate. The condensed heavy phase 148 in decanter 141 willgenerally comprise methyl iodide, methyl acetate, and PRCs. Thecondensed heavy liquid phase 148 in the decanter 141 can be convenientlyrecirculated, either directly or indirectly, to the reactor 130. Forexample, a portion of this condensed heavy liquid phase 148 can berecirculated to the reactor, with a slip stream (not shown), generally asmall amount, e.g., from 5 to 40 mol. %, or from 5 to 20 mol. %, of theheavy liquid phase being directed to a PRS. This slip stream of theheavy liquid phase 148 may be treated individually or may be combinedwith the condensed light liquid phase 147 for further distillation andextraction of carbonyl impurities in accordance with one embodiment ofthe present invention.

As shown in FIGS. 3-5, the light phase exits decanter 141 via stream147. A first portion, e.g., aliquot portion, of light phase stream 147may be recycled to the top of the light ends column 140 as a refluxstream. A second portion, e.g., aliquot portion, of light phase stream147 may be directed to the optional PRS (not shown), e.g., as shown bystream 149. A third portion, e.g., aliquot portion, of the light phasestream 147 optionally may be recycled to reactor 130, e.g., as shown byoptional recycle stream 150, when additional water is desired or neededin reactor 130. In preferred aspects the water level in the reactor ismaintained at a desired level without recycling stream 150 to reactor130 since recycling stream 150 to the reactor 130 undesirably willresult in the recycle of acetic acid and unnecessarily increasing theload on reactor 130.

Light ends column 140 also preferably forms a residuum or bottoms stream145, which comprises primarily acetic acid and water. Since light endsbottoms stream 145 typically will comprise some residual catalyst, itmay be beneficial to recycle all or a portion of the light ends bottomsstream 145 to reactor 130, as shown by optional line 150. Preferably,the light ends bottoms stream 145 may be combined with catalyst phase138 from flasher 131 and returned together to reactor 130.

As indicated above, in addition to the overhead phase, the light endscolumn 140 also forms an acetic acid side stream 144, which preferablycomprises primarily acetic acid and water. Optionally, a portion of sidestream 144 may be recirculated to the light ends column, preferably to apoint below where side stream 144 was removed from light ends column, asdescribed in US Pub. No. 2008/0287706, the entirety of which is herebyincorporated by reference.

Since side stream 144 contains water in addition to acetic acid, sidestream 144 from the light ends column 140 preferably is directed to adrying column 142, in which the acetic acid and water are separated fromone another. As shown, drying column 142, separates acetic acid sidestream 144 into an overhead stream 151 comprised primarily of water anda bottoms stream 152 comprised primarily of purified, dried acetic acid.Overhead stream 151 preferably is cooled and condensed in a phaseseparation unit, e.g., decanter 153, to form a light phase 154 and aheavy phase 155. As shown, a portion of the light phase 154 is refluxed,as shown by stream 156 and the remainder of the light phase is returnedto the reactor 130, as shown by stream 157. The heavy phase, whichtypically is an emulsion comprising water and methyl iodide, preferablyis returned in its entirety to the reactor 130, as shown by stream 155,optionally after being combined with stream 157, although a portion mayalso be further processed (not shown).

The drying column bottoms stream 152 preferably comprises or consistsessentially of acetic acid. In preferred embodiments, the drying columnbottoms stream comprises acetic acid in an amount greater than 90 mol %,e.g., greater than 95 mol % or greater than 98 mol %. Optionally, thedrying column bottoms stream 152 may be further processed, in a heavyends column (not shown) or iodide guard bed (not shown), prior to beingstored or transported for commercial use. Of course, the separationsystems of FIGS. 3-5, are merely examples of separation schemes that maybe utilized in the present invention. Other combinations of separationunits may just as easily be utilized. Preferable separation systems arethose wherein at least a portion of residual carbon monoxide isseparated and/or recovered from the crude acetic acid product.

Returning to vent gas stream 146 from the overhead decanter 141 of thelight ends column 140. In a preferred embodiment, vent gas stream 146,which comprises an amount of residual carbon monoxide, may be directedto secondary reaction zone 103. In FIGS. 3 and 4, the vent gas stream146 is initially processed in a recovery unit 160 to remove any lowboiling point compounds, such as methyl iodide.

A scrubbing solvent 161, preferably chilled to less than 25° C., may befed to recovery unit 160 to scrub vapor stream 146 of low boiling pointcomponents, such as methyl iodide, which are removed via line 162 andare preferably returned to the reaction zone 101. Scrubbing solventsinclude methanol, methyl acetate, dimethyl ether, acetic acid andmixtures thereof. The overheads of recovery unit 160 may exited as purgegas 163. In one optional embodiment, a portion of the vent gas stream146 may by-pass the recovery unit 160 in line 164 and be combined withthe overhead vapor 163.

Scrubbed gas 163 exiting the top of recovery unit 160 comprises carbonmonoxide, methyl acetate, and optionally methyl iodide. In preferredembodiments, scrubbed gas 163 passes through compressor 165 to form highpressure derivative stream 166. In preferred embodiments, the totalpressure of the vapor in high pressure derivative stream 166 is from 0.1MPa to 10 MPa, e.g., 0.5 MPa to 5 MPa or 0.5 MPa to 2 MPa. In oneembodiment, the scrubbed gas comprises substantially no methyl iodide,which has been removed by recovery unit 160. High pressure derivativestream 166 is fed to the secondary reaction zone 103 in FIG. 3. Thesecondary reaction zone 103 comprises the secondary reactor 111 asdescribed above in FIG. 1. Secondary crude product stream 116 isreturned to separation zone 102 and co-fed with the side stream 144 todrying column 142. Derivative stream 110 from the heavy phase stream 148may be also fed to the secondary reactor 111 with the liquid stream 119.

In FIG. 4, vent gas stream 146 from the overhead decanter 141 passesthrough the recovery unit 160 to generate scrubbed gas 163. In oneembodiment, scrubbed gas 163 does not pass through a compressor andremains at a relatively lower pressure than high pressure derivativestream 166 in FIG. 3. Scrubbed gas 163 is fed to secondary reaction zone103. The secondary reaction zone 103 of FIG. 4 comprises the secondaryreactor 112 as described above in FIG. 2. Optionally, along with thescrubbed gas 163, fresh reactants in line 115 and derivative stream 110,may also be fed to secondary reactor 112. Secondary reactor 112generates a secondary crude product stream 125 that is condensed and fedto a knock-out pot 126 to remove a liquid stream 127 comprising aceticacid. Liquid stream 127 is returned to separation zone 102 and co-fedwith the side stream 144 to drying column 142.

In FIG. 5, vent gas stream 146 is fed to secondary reaction zone 103. Inone embodiment, vent gas stream 146 from the overhead decanter 141 maypass through a compressor 167 to form high pressure derivative stream168 that is fed directly to a secondary reactor 111 as taught in FIG. 5.In preferred embodiments, the total pressure of the vapor in highpressure derivative stream 168 is from 0.1 MPa to 10 MPa, e.g., 0.5 MPato 5 MPa or 0.5 MPa to 2 MPa. High pressure derivative stream 168 maycomprise a higher amount of methyl iodide relative to the high pressurederivative stream 166 taught in FIG. 3. In alternative embodiments, thevent gas stream 146 may be fed as the derivative stream to a secondaryreactor, without being compressed or processed in a recovery unit.

As discussed above, the present invention utilizes the carbon monoxidederivative stream(s) to yield additional acetic acid which improvesoverall process efficiency. FIGS. 1-5 show various derivative streamsthat may be utilized in the secondary reaction of the present invention.Of course, other derivative streams that contain carbon monoxide mayalso be utilized in accordance with the present invention.

In one embodiment, the derivative stream may be derived from an aceticacid production process as described in CN 101439256A, which isincorporated herein by reference. In one embodiment, the process mayinvolve 1) carbonylation of methanol in a carbonylation reactor to formacetic acid and 2) hydrogenating the acetic acid in a hydrogenationreactor with hydrogen derived, at least in part, from a tail gas streamformed either in the carbonylation process or in the hydrogenationprocess. Carbon monoxide may also be recovered from the tail gas. In oneinstance, the tail gas comprising carbon monoxide and hydrogen may bederived from the crude acetic acid stream and/or the crude ethanolstream. This tail gas may be employed in accordance with the presentinvention to make additional acetic acid. Of course, other carbonmonoxide-containing derivative streams in the system may also beemployed.

In another embodiment, the derivative stream may be derived from anacetic acid process as described in CN1537840, which is incorporatedherein by reference. This process may utilize a reaction zone and aseparation zone. In some embodiment, the separation zone comprises interalia a column for separating water, methyl iodide, and acetic acid; aphase separator; and a rectifying column. Any suitable carbonmonoxide-containing streams, e.g., vent streams, from these processesmay be employed in accordance with the present invention to makeadditional acetic acid.

In another embodiment, the derivative stream may be derived from anacetic acid process as described in U.S. Pat. No. 5,334,755, which isincorporated herein by reference. This process may utilize a reactionzone and a separation zone. In some embodiment, the separation zonecomprises inter alia a heat exchanger, a distillation column, and/or agas-liquid separator. Any suitable carbon monoxide-containing streams,e.g., vent streams, from these processes may be employed in accordancewith the present invention to make additional acetic acid.

In one embodiment, the derivative stream may be derived from an aceticacid production process as described in U.S. Pat. No. 6,140,535, whichis incorporated by reference herein. This acetic acid production processreacts carbon monoxide with methanol and/or a methanol derivative in aliquid reaction composition comprising (i) an iridium carbonylationcatalyst, (ii) a methyl iodide co-catalyst, (iii) a metal promoterselected from the group consisting of ruthenium, osmium, rhenium,cadmium, mercury, zinc, gallium, indium and tungsten; (iv) a finiteamount of water at a concentration of less than about 8% by weight; (v)methyl acetate; (vi) acetic acid; and (vii) propionic acid by-productand its precursors. The process further comprises the step ofwithdrawing liquid reaction composition from the carbonylation reactorand introducing at least part of the withdrawn liquid reactioncomposition, with or without the addition of heat, to a flash zone toform a vapor fraction comprising water, acetic acid product, propionicacid by-product, methyl acetate, methyl iodide and propionic acidprecursors, and a liquid fraction comprising involatile iridiumcatalyst, involatile optional promoter or promoters, acetic acid andwater. The vapour outlet from the flash zone is connected to a firstdistillation zone provided with an overhead condenser and a decanter. Inuse, the vapours from the distillation zone are condensed into thedecanter and form two phases, a methyl iodide-rich phase and an aqueousphase. The heavy methyl iodide rich phase is recycled to thecarbonylation reactor and the lighter aqueous phase is divided; partbeing used as reflux to the distillation zone and part being recycled tothe carbonylation reactor. In this system, any of the process stream(s)that may contain residual carbon monoxide, e.g., a vapor stream that maybe withdrawn from the decanter, may be utilized in the secondaryreaction zone of the present invention.

In one embodiment, the derivative stream may be derived from an aceticacid production process as described in U.S. Pat. No. 5,391,821, whichis incorporated herein by reference. This process may utilize a reactionzone and a separation zone. In some embodiment, the separation zonecomprises inter alia a column for separating water, methyl iodide, andacetic acid; a phase separator; and a rectifying column. Any suitablecarbon monoxide-containing streams, e.g., vent streams, from theseprocesses may be employed in accordance with the present invention tomake additional acetic acid.

In one embodiment, the derivative stream may be derived from an aceticacid production process as described in U.S. Pat. No. 7,678,940, whichis incorporated herein by reference. This process produces a carboxylicacid and may utilize a reaction zone and a separation zone. In someembodiment, the separation zone comprises inter alia acatalyst-separating column and a first distillation column. An alcoholor an ester corresponding to the carboxylic acid, e.g., acetic acid, maybe fed to the first distillation column. The separation zone may furthercomprise a second distillation column, a condenser, and/or a phaseseparator. Any suitable carbon monoxide-containing streams, e.g., ventstreams, from these processes may be employed in accordance with thepresent invention to make additional acetic acid.

In one embodiment, the derivative stream may be derived from an aceticacid production process as described in Jones, J. H. (2002), “TheCativa™ Process for the Manufacture of Acetic Acid,” Platinum MetalsReview, 44 (3): 94-105 discussed above. Any suitable carbonmonoxide-containing streams, e.g., vent streams, from these processesmay be employed in accordance with the present invention to makeadditional acetic acid.

In other embodiments, the derivative stream may be derived from anacetic acid production processes as described in Noriyuki Yoneda andYasuo Hosono (2004); “Acetic Acid Process Catalyzed by IonicallyImmobilized Rhodium Complex to Solid Resin Support;” Journal of ChemicalEngineering of Japan, Vol. 37, No. 4, the disclosure of which is herebyincorporated by reference. These acetic acid production processes employan ionically immobilized homogeneous rhodium catalyst in a methanolcarbonylation process, e.g., the Monsanto process.

While the invention has been described in detail, modifications withinthe spirit and scope of the invention will be readily apparent to thoseof skill in the art. In view of the foregoing discussion, relevantknowledge in the art and references discussed above in connection withthe Background and Detailed Description, the disclosures of which areall incorporated herein by reference. In addition, it should beunderstood that aspects of the invention and portions of variousembodiments and various features recited below and/or in the appendedclaims may be combined or interchanged either in whole or in part. Inthe foregoing descriptions of the various embodiments, those embodimentswhich refer to another embodiment may be appropriately combined withother embodiments as will be appreciated by one of skill in the art.Furthermore, those of ordinary skill in the art will appreciate that theforegoing description is by way of example only, and is not intended tolimit the invention.

We claim:
 1. A process for producing acetic acid, comprising the stepsof: (a) reacting a carbon monoxide feed comprising: (i) less than 99.5mol % carbon monoxide; and (ii) at least one of methanol and a methanolderivative in a first reactor under conditions effective to produce acrude acetic acid product; (b) purging from the first reactor a carbonmonoxide purge stream comprising a first amount of residual carbonmonoxide and one or more impurities, the first amount being greater than20 mol %; (c) separating the crude acetic acid product into at least onederivative stream, at least one of the at least one derivative streamcomprising a second amount of residual carbon monoxide; and (d) reactingat least a portion of the second amount of residual carbon monoxide andat least one of methanol and a methanol derivative in a second reactorunder conditions effective to produce additional acetic acid.
 2. Theprocess of claim 1, wherein the overall carbon monoxide conversion isgreater than 95%, based on the total amount of carbon monoxide in thecarbon monoxide feed.
 3. The process of claim 1, wherein at least aportion of the carbon monoxide in the purge stream is reacted with atleast one of methanol and a methanol derivative in the second reactorunder conditions effective to produce additional acetic acid.
 4. Theprocess of claim 1, further comprising the step of: directing at least aportion of the purge stream to a purification zone configured downstreamof the first reactor, wherein the at least a portion of the purge streamcomprises at least 20 mol % carbon monoxide.
 5. The process of claim 1,further comprising the step of: directing at least a portion of thepurge stream to a flasher, wherein the at least a portion of the purgestream comprises at least 20 mol % carbon monoxide.
 6. The process ofclaim 1, further comprising the step of: directing at least a portion ofthe purge stream to a light ends column, wherein the at least a portionof the purge stream comprises at least 20 mol % carbon monoxide.
 7. Theprocess of claim 1, wherein the at least one derivative streamcomprising residual carbon monoxide, comprises: from 10 mol % to 95 mol% carbon monoxide; and from 5 mol % to 90 mol % at least one of methanoland a methanol derivative.
 8. The process of claim 1, wherein thepurging maintains the carbon monoxide partial pressure in the reactor ata predetermined level ranging from 0.13 MPa to 1.1 MPa.
 9. The processof claim 1, wherein the purge stream comprises: carbon monoxide in anamount ranging from 20 mol % to 95 mol %; nitrogen in an amount rangingfrom 0.5 mol % to 20 mol %; methane in an amount ranging from 0.5 mol %to 20 mol %; carbon dioxide in an amount ranging from 0.5 mol % to 30mol %; and hydrogen in an amount ranging from 0.5 mol % to 50 mol %. 10.The process of claim 1, wherein the reaction temperature in the secondreactor ranges from 100° C. to 350° C.
 11. The process of claim 1,wherein the reaction pressure in the second reactor ranges from 0.01 MPato 10 MPa.
 12. The process of claim 1, wherein the second reactorcomprises a solid metal catalyst comprising at least one metal selectedfrom the group consisting of rhodium, iridium, ruthenium, nickel, andcobalt.
 13. The process of claim 1, wherein the at least one of methanoland a methanol derivative employed in the second reactor is methylacetate.
 14. A process for producing acetic acid, comprising (a)reacting a carbon monoxide feed comprising (i) less than 99.5 mol %carbon monoxide and (ii) at least one of methanol and a methanolderivative in a first reactor under conditions effective to produce acrude acetic acid product; (b) separating the crude acetic acid productinto at least one derivative stream, at least one of the at least onederivative stream comprising residual carbon monoxide; and (c) reactingat least a portion of the residual carbon monoxide and at least one ofmethanol and a methanol derivative in a second reactor under conditionseffective to produce additional acetic acid.
 15. The process of claim14, wherein the overall carbon monoxide conversion is at least 90%. 16.The process of claim 14, wherein the carbon monoxide feed comprises lessthan 95 mol % carbon monoxide.
 17. The process of claim 14, wherein thecarbon monoxide feed comprises from 10 mol % to 99.5 mol % carbonmonoxide.
 18. The process of claim 14, wherein the carbon monoxide feedcomprises less than 60 mol % carbon monoxide.
 19. The process of claim14, wherein the carbon monoxide feed comprises from 60 mol % to 70 mol %carbon monoxide.
 20. A process for producing acetic acid, comprising (a)reacting a carbon monoxide feed comprising: (i) less than 99.5 wt %carbon monoxide and (ii) at least one of methanol and a methanolderivative in a first reactor under conditions effective to produce acrude acetic acid product; (b) withdrawing from the first reactor apurge stream comprising impurities and a first amount of residual carbonmonoxide; (c) separating the crude acetic acid product into at least onederivative stream, at least one of the at least one derivative streamcomprising as second amount of residual carbon monoxide; and (d)reacting the first and second amounts of residual carbon monoxide and atleast one of methanol and a methanol derivative in a second reactorunder conditions effective to produce additional acetic acid.
 21. Theprocess of claim 20, wherein the purge stream is combined with the atleast one derivative stream to form a combined stream and the combinedstream is directed to the second reactor.
 22. The process of claim 20,wherein less than 10 mol % of the first and second amounts of residualcarbon monoxide, in total, are vented from the process.